http: emit 'drain' on OutgoingMessage only after buffers drain
Previously, socketOnDrain could be invoked synchronously from _flushOutput (via _onPendingData -> updateOutgoingData) while the bytes just handed to the socket were still buffered and while outputSize had not yet been reset on the OutgoingMessage. The 'drain' event fired even though res.writableLength was non-zero, breaking the invariant a user would reasonably expect after while (!res.write(...));.
Gate the emission in socketOnDrain on msg.writableLength === 0 (which also covers outputSize + chunked buffer + socket.writableLength), and apply the same check in OutgoingMessage._flush so that 'drain' is only emitted when the response is genuinely drained. The socket's own 'drain' event will otherwise propagate through socketOnDrain when the socket buffer actually empties.
Assisted-by: Claude Opus 4.6 (1M context) noreply@anthropic.com